Children with spina bifida who are wheelchair-users are less physically active than their typically developing peers. It is important to understand relations between physical activity and other factors, so approriate interventions can be developed. We explored relations between physical activity and fitness, age, sex and severity of disability in children that were 5–19 years of age who were diagnosed with spina bifida and who are wheelchair-users. We found that older age and the inability to walk negatively influence physical activity. We did not find a relation between physical activity and fitness or physical activity and sex.